This invention relates to a method and device for directly crystallizing a monocrystal without using a seed crystal. This is achieved by first supercooling a melt of molten raw material which is floating under microgravitational conditions, then generating a crystal nucleus, and solidifying the melt. This invention relates to an art which is particularly suitable for the manufacture of granular monocrystals of single element semiconductors and granular monocrystals of compound semiconductors.
Single element semiconductor crystals, such as silicon or germanium or the like, two-element compound semiconductor crystals, such as GaAs, GaP, GaSb, InAs, InP, InSb, ZnSe, CdTe, or the like, or mixed crystals in which 2 two-element compound semiconductors are mixed have been used as materials for electronics devices. The quality of these semiconductor crystals has a large effect on device performance. As a result, the art of manufacturing high quality bulk monocrystal (monocrystal), having few crystal defects and having a controlled composition ratio of the component elements and impurity concentration distribution, is extremely important.
If monocrystals can be manufactured by directly synthesizing a chemical compound melt from the semiconductor raw material and directly solidifying this melt without using seed crystals, the performance of the electronics device can be improved. This is also preferable in terms of the manufacturing cost. Presently, known methods in which large bulk monocrystals are grown by solidifying melt of semiconductor raw material include CZ method, FZ method, Bridgeman method, and the like.
However, with all of these methods, the monocrystal is grown from a seed crystal, and as a result, a good quality seed crystal must be prepared first. Generally, the method which is adopted is a method in which the seed crystal is cut from a large monocrystal which is manufactured separately. However, depending on the type of semiconductor, it can be difficult to manufacture a good quality monocrystal. In these cases, a sintered body or a precious metal rod is used as a seed, and a polycrystal is grown. A monocrystal portion with a comparatively large grain is cut from this polycrystal and made into the seed crystal. However, it is difficult to obtain a good quality seed crystal.
In the method for growing crystals from a melt of the prior art, complex fluid motion is generated within the melt due to the influence of gravity, and it is known that this can have a large effect on the quality of the growing crystal. As its most important drawback, a crucible for storing the melt is needed. A reduction in the purity of the crystal or defects in the crystal are generated because of the chemical physical actions of the crucible. Thermal convection is generated when there are temperature differences within the melt. Because of the resulting fluctuation in the temperature and composition at the solid/liquid interface, crystal defects are readily generated, and quality is unstable. This results in a crystal with non-uniform composition and many crystal defects.
In order to eliminate the negative effect of gravity, various experiments have been conducted on growing crystals under microgravitational conditions which are achieved in space stations, space shuttles, rockets, and aircrafts. However, the costs for crystal manufacture becomes enormous, and the materials which can be used become limited. In addition, because of minute gravitational disturbances called G jitters, shaking is generating during the crystal growth period. This is not ideal. Recently, a free-fall facility located on the ground has achieved microgravitational conditions with few G jitters, although only for the short period of time of approximately 10 seconds. There is much anticipation on its use, but methods for directly growing monocrystals from melts have not been proposed.
For example, in U.S. Pat. No. 4,021,323, there is described a technology, wherein: molten silicon droplet is shot from a small nozzle which is placed on the upper end of a shot tower; silicon melt is allowed to free fall in air from the shot tower, and granular crystals of silicon are created. However with this technology, an adequate microgravitational environment is not created due to the air resistance during the fall. In addition, impurities from the nozzle may become dissolved into the molten silicon.
The present invention is based on new facts discovered by the present inventors while conducting various experiments of creating spherical crystals from semiconductor melt under microgravitational conditions generated by a free fall facility.